There are various processes that utilize radiation—e.g., sterilization, radiation therapy, food irradiation, quality checking, etc.—and these processes have a need to verify the radiation dose. Similarly, there is a large number of different methods to determine a dose—e.g., ion dosimetry (ionization in air), calorimetry (determination of heat in carbon or metals), thermoluminescence dosimetry (luminescence in solids), etc. The formation of radicals in solid organic substances on irradiation has been observed and the concentration of these radicals is proportional to the absorbed dose over a wide range. The concentration of the radicals can be determined easily by means of electron paramagnetic resonance (EPR) spectroscopy. Alanine has been widely used for this purpose due to its availability and the relative simplicity of incorporating it into practical dosimeters. An advantage of the use of organic materials such as alanine over inorganic dosimeter systems is that it can be assumed that the irradiation-induced changes in organic materials are closer to radiation effects in living tissues.
Alanine dosimetry is an accepted method to determine the radiation dose of different irradiation processes. On irradiating with ionizing radiation, radicals will be produced in alanine which are stable for long periods. This is mainly due to the inhibition of radical-radical recombinations in the crystalline structure of the material that prevents the migration of large molecule fragments. The non-destructive evaluation of the radical concentration can be done using EPR spectroscopy. The determination of irradiation doses by means of EPR techniques requires a sensitive, robust and reliable instrument that can be served by a laboratory worker. A useful instrument provides such features as automated procedures for calibration and measurements. Careful adjustment of the EPR spectrometer and the selection of suitable dosimeters allows the determination of dose rates in a range from 2 Gy to 200 kGy with a total uncertainty of 3.5% (confidence level of 95%). Alanine dosimeters are small, stable, and easy to handle. They are characterized by their large measuring range and a low sensitivity to temperature and humidity. This allows for their application in radiation therapy, the irradiation of blood, as well as in industrial facilities for irradiation. The dosimeter system can be used for reference and routine dosimetry due to its high quality and low costs.
Alanine dosimeters are well known in the art. For example, in the reference: T. Kojima et al., “Alanine Dosimeters Using Polymers As Binders”, Applied Radiation & Isotopes, vol. 37, No. 6, (1986), Pergamon Journals Ltd., pp. 517-520, there are numerous references to dosimeters made in pellet, rod, and film formats. Prominent among these references are “A Polymer Alanine Film for Measurements of Radiation Dose Distributions”, Appl. Radiat. Isot. Vol. 39 (7) pp. 651-657, 1988 and “Dosimetry for Cobalt-60 Gamma Rays with Alanine”, Radiation Protection Dosimetry, vol. 9 (4) pp. 277-281 1984. Dosimeters have been made both by industrial laboratories and at academic institutions. Many of these dosimeters are in the form of molded pellets or rods. The alanine is generally blended with a synthetic or natural rubber, compounded and molded under pressure to form a variety of shapes (U.S. Pat. No. 4,668,714, J.P. 203276 J.P. 0125085, J61057-878-8). There are also references in the literature to extruded films (J01102-388-A). These extruded products, while working well, have several deficiencies. Their manufacture often requires the use of high pressures and temperatures during the molding process, requiring molding equipment that limits the sizes and shapes available. Molded dosimeters are also limited in that only moldable polymeric binders may be used. The use of molded dosimeters is also somewhat restrictive, as the size of the dosimeters tends to be very small, leading to difficulties in handling and possibly loss during irradiation.
A potential solution to these difficulties would be an alanine dosimeter coated onto a flexible support wherein the support serves not only to hold the alanine, but also provides the user with a length and width that allow easy handling. Such a coated dosimeter has been described in DE 19637471 A. In this art, the alanine is coated from two, specific binders—a polyoctenamer or polystyrene. Both of these binders are brittle materials and make the coating of thick alanine layers with good mechanical properties very difficult, especially when the thickness of the dosimeter layer is >100 microns. The ability to bend and shape the alanine dosimeter coated on to the plastic support can be very important in some applications, and is a significant limitation of the coated dosimeters described in the art. U.S. application Ser. No. 09/995,080 describes coated dosimeters using elastomeric binders, which overcome this limitation to provide dosimeters that are highly flexible.
Several types of radiation are used in industry to effect sterilization or in materials processing. Gamma radiation, especially that from Cobalt 60, is very commonly used in the irradiation of food stuffs. Electron beam (e-beam) technologies are used in the irradiation of foods, but are also used in the sterilization of medical device and the crosslinking of polymeric materials to improve durability and chemical resistance. One problem with dosimeters known in the art, when used with electron beam irradiation, is the accumulation of charge on the surface of the dosimeter. This charge, which is in the form of free electrons, is read as a signal by the EPR spectrometer and gives a false indication of the dose received by the irradiated object. While the charge does eventually dissipate, the length of time between irradiation and the time at which the dosimeter is free of charge and indicates a stable reading can be several hours and can be a major inconvenience to the irradiating facility.
It would be useful in the industry to have a dosimeter that dissipates electron beam induced charge quickly so that accurate dosimetry readings could be made shortly after irradiation.